1. Field of the Invention
The invention relates to a process for producing lube oil fractions by solvent extraction. It more particularly relates to an improvement to such process whereby a voltage gradient is maintained in the extract phase.
2. Discussion of the Prior Art
Solvent extraction is a well established process used in the refining of petroleum, the first application having been made in about 1911. Originally, upgrading of kerosine was a major use but the improvement obtained in solvent extracting lubricating oils and other products soon became an important application. Solvent extraction is used extensively in the petroleum refining industry to refine lubricating oils, kerosine and specialty oils for medicinal and agricultural purposes. It is also used for upgrading charge stocks for catalytic cracking operations as well as for separating light aromatics from gasoline. Solvent extraction is a process that separates hydrocarbon mixtures into two phases; a raffinate phase which contains substances of relatively high hydrogen to carbon ratio, often called paraffinic type materials, and an extract phase which contains substances of relatively low hydrogen to carbon ratio, often called aromatic type materials. Therefore, it may be said that solvent extraction is possible because different liquid compounds have different solution affinities for each other, and some combinations are completely miscible while other combinations are almost immiscible. The ability to distinguish between high carbon to hydrogen aromatic type and low carbon to hydrogen or paraffinic type materials is termed selectivity. The more finely this distinguishing can be done the higher the selectivity of the solvent.